Such valves for spray guns are as a rule formed by a valve housing and a valve element or valve piston disposed in the valve housing. Located in the valve piston is a longitudinal bore which serves as a mixing chamber. Furthermore, transverse bores form supply channels in the valve piston. The valve piston is mounted so as to be longitudinally mobile in the valve housing and can be moved back and forth in the longitudinal direction, for example by means of a further, pneumatic, double-acting piston. In the course of the motion of the valve piston, the supply channel or channels arrive(s) in a position in alignment with a material supply line through which a pressurised spray medium is conveyed into the supply channel and subsequently into the mixing chamber. A further movement of the valve piston leads to closing of the supply channel through, for example, the valve housing and subsequently to opening of the supply channel in a second position, so that compressed air can flow into the supply channel and the spray medium present in the supply channel and in the mixing chamber can be conveyed out of the mixing chamber and atomised or sprayed into the surroundings. For example, two supply channels can be provided, via which one component of a two-component paint can in each case be supplied. Since an alternating motion of the valve piston takes place at a relatively high speed, a virtually uniform delivery of, for example, a paint can take place via the valve and the respective spray gun.
The described arrangement of a valve piston in a cylinder or a valve housing and the relative motion of the valve piston along its longitudinal axis has a number of drawbacks. Since it is not only the valve piston that has to be moved, but also a piston of a pneumatic cylinder for driving the valve piston, a relatively large mass has to be moved, which leads to undesired vibration of a spray gun in operation. Moreover, there is a need for numerous seals in order to seal the valve piston in the valve housing, as well as in the region of the pneumatic cylinder. The rapid, linear motion of the valve piston creates premature wear of the seals and moreover requires sufficient lubrication. In addition, such a valve can only be cleaned with difficulty, since the valve piston has to be connected fixedly to its linear drive. As a rule, therefore, the known valves are therefore designed together with a pneumatic cylinder as a spray gun, i.e. they form a common component due to the formation of a common housing. A replacement of the valve is not therefore possible and the dismantling of the valve piston for cleaning gives rise to an expensive assembly outlay.